One of the most critical problems associated with a rotary internal combustion engine is leakage at the seal grid of a rotor. Highly efficient dynamic sealing is mandatory between apices of the rotor and its surrounding housing if the engine is to have performance and efficiency better than current commercial automative engines. Various factors contribute to the lack of an adequate solution in this area: housing distortion due to wide variations in local operating temperature, the gas-actuated apex seal loses effectiveness at points where the source of gas pressure shifts its orientation with respect to the seal, and the factor that the apex seal is alternately dragged and pushed against the rotor housing during different quadrants of movement. Seal grid leakage ultimately affects cranking efficiency, speed, fuel economy, low-end torque and unburned hydrocarbon emission levels.
Prior art seal constructions to date have typically comprised a strip of material, such as cast iron or graphite, received in a transverse slot at each of the apices of the rotor; each strip has a curved crown to make a line contact with the rotor housing. The strip is urged into engagement with the rotor housing by a combination of three forces: a mechanical spring working radially outwardly against the base of the strip, centrifugal force, and combustion gas pressure. The latter is the most predominant force that affects sealing. Sealing is designed to take place along the crown line contact and along a line or surface contact at one side of the strip with one side of the slot. The lateral tolerance between the strip and slot has been arranged with some looseness requiring the strip to shift from one slot side to the other to effect a new sealing mode as pressure shifts during a full revolution of the rotor. High gas pressure, performing as the workhorse among the three seal forces, will be on different sides of the strip at different quadrants of rotor movement.
Various attempts have been made to solve the leakage problem by a metallurgical approach which has involved substitution of a variety of materials to obtain more stable operating conditions. Although some improvement has been noted by this approach, it is now more widely acknowledged that a solution, if there is one, resides in a mechanical design approach. To this end, prior art mechanical attempts (such as U.S. Pat. No. 3,172,767) have included making a three-piece seal strip with 45.degree. angled surfaces between mating ends of the pieces thereby allowing the seal strip to laterally accommodate different dimensions between the housing side walls abutting the open ends of the slots. However, the mechanical spring must act against the remote end pieces, leaving the center piece without the same radial forces operating to urge it against the rotor housing. This can result in a considerable gap or slit between the crown of the center piece and the trochoid surface when a pressure relief may occur during seal shifts, thus allowing gas leakage to dramatically reduce efficiency of the engine.
The prior art has also made some attempt to overcome leakage during seal shifts, or more accurately, reduce the time lag for gas pressure to shift the seal within the slot. It has been generally accepted by one approach that cocking or skewing of the apex seal strip within the slot is a necessary phenomenon; therefore non-symmetrical passages, communicating with the bottom side of the seal, are useful to promote gas communication. This construction is further discussed in the detailed description, but suffice it to say that it is not successful in reducing the time lag to avoid gas leakage.
Still another prior art approach (illustrated in U.S. Pat. No. 3,176,909) to solving the time lag problem, also detailed in the specification, has been to provide a shuttle element immediately below the apex seal strip which in turn is spring urged to act against the apex seal for sealing with the rotor housing. Slots are provided in the rotor penetrating through the sides of the seal slot to communicate gas pressure to the shuttle and thereby force the shuttle transversely to promote a seal between the sides of the shuttle and the slot. However, due to spring friction against the shuttle and the large surface contact between the shuttle and the strip itself, there has been no reduction of the time lag for the seal shift. In fact, it has been hindered by this particular construction.
Yet still another approach is illustrated in U.S. Pat. No. 3,171,587, which provides openings in the rotor body to communicate gas pressure with the mid-section of the leading and trailing seal strip side walls, the seal strip being unmodified. This is disadvantageous because the mass of the seal strip remains unimproved, the orifice area through which gas pressure communicates with the underside of the seal remains the same while the length of said orifice is undesirably variable depending on the quadrant position of the rotor. The unit pressure force holding said seal against a slot side wall is slightly improved. In another embodiment of this patent, the seal strip body and crown have small recesses extending above the rotor slot walls which communicate directly with chamber pressure. This is disadvantageous because the recesses must be made inherently small to leave sufficient body metal as a guide to maintain the body upright in the slot; thus body mass remains substantially the same, unit pressure holding the seal against the wall remains low, and the flow orifice is insufficient if the walls between lands defined by the recesses as well as the lands are considered as the throat walls. In addition, leakage paths are promoted by placing the recesses in a position straddling camming surfaces (during momentary seal shifts); when the underside pressure does not remain high enough, the camming surfaces will part slightly).